Method of evaluating high fatigue strength material in high tensile strength steel and creation of high fatigue strength material

Abstract

The present invention provides a method of designing a high fatigue strength in high tensile strength steel, comprising: obtaining values of tensile strength sigmaB (unit thereof is MPa) and Vickers hardness Hv of the steel; measuring a flaw area of an inclusion, when a fracture origin is located only at a surface of the steel; and estimating, in designing the high fatigue strength steel, that a fatigue limit sigmaw (unit thereof is MPa) of the steel satisfies either sigmaw>=0.5 sigmaB or sigmaw>=1.6 Hv, when a square root of the flaw area, (area)½ (unit thereof is m), contained in the steel is no larger than 45.8/sigmaB2 or 4.47/Hv2. According to the present invention, a method of evaluating high fatigue strength in high tensile strength steel, in which method a relationship between a flaw dimension (area) of ODA and the fatigue strength is considered, and a high fatigue strength material can be provided.

What is claimed is:  
     
       1. A method of designing a high fatigue strength metal, comprising: 
       obtaining a value σ B , corresponding to a tensile strength of a metal in terms of MPa;  
       obtaining a value Hv, corresponding to a Vickers hardness of said metal; and  
       estimating a fatigue limit σ w , of said metal in terms of MPa to satisfy one of the following equations  
       
         
           σ w ≧0.5 σ B ,  (i)  
         
       
       and 
       
         
           σ w ≧1.6 Hv  (ii)  
         
       
        when a fracture origin is located only at a surface of said metal, and when a square root of a flaw area contained in said metal is not greater than 458/σ 2   B  or 4.47/Hv 2 .  
     
     
       2. The method according to  claim 1  wherein said metal comprises steel. 
     
     
       3. The method according to  claim 2  wherein said flaw area corresponds to a cross-sectional flaw area. 
     
     
       4. A method of designing a high fatigue strength metal, comprising: 
       obtaining a value σ B  corresponding to a tensile strength of a metal in terms of MPa;  
       obtaining a value Hv corresponding to a Vickers hardness of said metal;  
       measuring a flaw area A of an inclusion of said metal when a fracture origin is located inside said metal; and  
       estimating a fatigue limit σw of said metal in terms of Mpa to satisfy the equation  
       
         
           σ w ≧3.38 A −¼ .  
         
       
     
     
       5. The method according to  claim 4  wherein said metal comprises steel. 
     
     
       6. The method according to  claim 5 , wherein said flaw area A corresponds to a cross-sectional area of said inclusion. 
     
     
       7. A method of evaluating a high tensile strength structure, which method can be used in designing a high fatigue strength metal, said method comprising: 
       measuring a maximum inhomogeneous elemental area B of a high fatigue strength structure after said high fatigue strength structure has been  
       (i) made otherwise homogeneous by limiting the inhomogeneous elemental area, or  
       (ii) minuturized by reducing a block width thereof;  
       when said high fatigue strength structure has been made otherwise homogeneous by limiting the inhomogeneous elemental area, setting a distribution of a maximum-minimum range of said maximum inhomogeneous elemental area B in terms of μm within a range defined by the lines  
       (a) B ½ =0, and  
       (b) B ½ =0.9403y+4.571, wherein y is a standardizing parameter and a test standard area is 6.2×10 −9  m 2 ; and  
       when said high fatigue strength structure has been miniaturized by reducing a block width thereof, setting a distribution of a maximum-minimum range of the block width d in terms of μm within a range defined by the lines  
       (c) d=0, and  
       (d) d=0.217y+0.701, wherein y is a standardizing parameter and a test standard area 1.0×10 −10  m 2 .  
     
     
       8. The method according to  claim 7 , wherein said metal comprises steel. 
     
     
       9. A method of producing a high fatigue strength metal, comprising: 
       designing a high fatigue strength metal by  
       (i) obtaining a value σ B  corresponding to a tensile strength of a metal in terms of MPa;  
       (ii) obtaining a value Hv corresponding to a Vickers hardness of said metal; and  
       (iii) estimating a fatigue limit σ w  of said metal in terms of MPa to satisfy one of the following equations  
       
         
           σ w ≧0.5 σ B ,  (a)  
         
       
       and 
       
         
           σ w ≧1.6 Hv  (b)  
         
       
        when a fracture origin is located only at a surface of said metal, and when a square root of a flaw area contained in said metal is not greater than 45.8/σ 2   B  or 4.47/Hv 2 ; and  
       evaluating a high tensile strength structure by  
       (iv) measuring a maximum inhomogeneous elemental area B of a high fatigue strength structure after said high fatigue strength structure has been  
       (a) made otherwise homogeneous by limiting the inhomogeneous elemental area, or  
       (b) minuturized by reducing a block width thereof;  
       (v) when said high fatigue strength has been made otherwise homogeneous by limiting the inhomogeneous elemental area, setting a distribution of a maximum-minimum range of said maximum inhomogeneous elemental area B in terms of μm within a range defined by the lines  
       (1) B ½ =0, and  
       (2) B ½ =0.9403y+4.571, wherein y is a standardizing parameter and a test standard area is 6.2×10 −9  m 2 ; and  
       (vi) when said high fatigue strength structure has been miniaturized by reducing a block width thereof, setting a distribution of a maximum-minimum range of the block width d in terms of μm within a range defined by the lines  
       (3) d=0, and  
       (4) d=0.217y+0.701, wherein y is a standardizing parameter and a test standard area is 1.0×10 −10  m 2 .  
     
     
       10. The method according to  claim 9  wherein said metal comprises steel. 
     
     
       11. The method according to  claim 10  wherein said flaw area corresponds to a cross-sectional flaw area. 
     
     
       12. A method of producing a high fatigue strength metal, comprising: 
       designing a high fatigue strength metal by  
       (i) obtaining a value σ B  corresponding to a tensile strength of a metal in terms of MPa;  
       (ii) obtaining a value Hv corresponding to a Vickers hardness of said metal;  
       (iii) measuring a flaw area A of an inclusion of said metal when a fracture origin is located inside said metal; and  
       (iv) estimating a fatigue limit σ w  of said metal in terms of Mpa to satisfy the equation σ w ≧3.38 A −¼ ; and  
       evaluating a high tensile strength structure by  
       (v) measuring a maximum inhomogeneous elemental area B of a high fatigue strength structure after said high fatigue strength structure has been  
       (a) made otherwise homogeneous by limiting the inhomogeneous elemental area, or  
       (b) minuturized by reducing a block width thereof; and  
       (vi) when said high fatigue strength has been made otherwise homogeneous by limiting the inhomogeneous elemental area, getting a distribution of a maximum-minimum range of said maximum inhomogeneous elemental area B in terms of μm within a range defined by the lines  
       (1) B ½ =0, and  
       (2) B ½ =0.9403y+4.571, wherein y is a standardizing parameter and a test standard area is 6.2×10 −9  m 2 ; and  
       (vii) when said high fatigue strength structure hag been miniaturized by reducing a block width thereof, setting a distribution of a maximum-minimum range of the block width d in terms of μm within a range defined by the lines  
       (3) d=0, and  
       (4) d=0.217y+0.701, wherein y is a standardizing parameter and a test standard area is 1.0×10 −10  m 2 .  
     
     
       13. The method according to  claim 12 , wherein said metal comprises steel. 
     
     
       14. The method according to  claim 13 , wherein said flaw area A corresponds to a cross-sectional area of said inclusion. 
     
     
       15. A method of producing a high fatigue strength metal, comprising: 
       designing a high fatigue strength metal by  
       (i) obtaining a value σ B  corresponding to a tensile strength of a metal in terms of MPa;  
       (ii) obtaining a value Hv corresponding to a Vickers hardness of said metal; and  
       (iii) estimating a fatigue limit σ w  of said metal in terms of MPa to satisfy one of the following equations  
       
         
           σ w ≧0.5 σ B ,  (a)  
         
       
       and 
       
         
           σ w ≧1.6 Hv  (b)  
         
       
        when a fracture origin is located only at a surface of said metal, and when a square root of a flaw area contained in said metal is not greater than 45.8/σ 2   B  or 4.47/Hv 2 ; and  
       subjecting said metal to a heating operation in a vacuum of at least 2×10 −6  Pa to temper said metal.  
     
     
       16. The method according to  claim 15  wherein said metal comprises steel. 
     
     
       17. The method according to  claim 16 , wherein said flaw area corresponds to a cross-sectional flaw area. 
     
     
       18. A high fatigue strength metal produced by: 
       designing a high fatigue strength metal by  
       (i) obtaining a value σ B  corresponding to a tensile strength of a metal in terms of MPa;  
       (ii) obtaining a value Hv corresponding to a Vickers hardness of said metal; and  
       (iii) estimating a fatigue limit σ w  of said metal in terms of MPa to satisfy one of the following equations  
       
         
           σ w ≧0.5 σ B ,  (a)  
         
       
       and 
       
         
           σ w ≧1.6 Hv  (b)  
         
       
        when a fracture origin is located only at a surface of said metal, and when a square root of a flaw area contained in said metal is not greater than 45.8/σ 2   B  or 4.47/Hv 2 ; and  
       evaluating a high tensile strength structure by  
       (iv) measuring a maximum inhomogeneous elemental area B of a high fatigue strength structure after said high fatigue strength structure has been  
       (a) made otherwise homogeneous by limiting the inhomogeneous elemental area, or  
       (b) minuturized by reducing a block width thereof;  
       (v) when said high fatigue strength has been made otherwise homogeneous by limiting the inhomogeneous elemental area, setting a distribution of a maximum-minimum range of said maximum inhomogeneous elemental area B in terms of μm within a range defined by the lines  
       (1) B ½ =0, and  
       (2) B ½ =0.9403y+4.571, wherein y is a standardizing parameter and a test standard area is 6.2×10 −9  m 2 ; and  
       (vi) when said high fatigue strength structure has been miniaturized by reducing a block width thereof, setting a distribution of a maximum-minimum range of the block width d in terms of μm within a range defined by the lines  
       (3) d=0, and  
       (4) d=0.217y+0.701, wherein y is a standardizing parameter and a test standard area is 1.0×10 −10  m 2 .  
     
     
       19. The high fatigue strength material according to  claim 18 , wherein said metal comprises steel. 
     
     
       20. The high fatigue strength metal according to  claim 19 , wherein said flaw area corresponds to a cross-sectional flaw area. 
     
     
       21. A high fatigue strength metal produced by: 
       designing a high fatigue strength metal by  
       (i) obtaining a value σ B  corresponding to a tensile strength of a metal in terms of MPa;  
       (ii) obtaining a value Hv corresponding to a Vickers hardness of said metal;  
       (iii) measuring a flaw area A of an inclusion of said metal when fracture origin is located inside said metal; and  
       (iv) estimating a fatigue limit σ w  of said metal in terms of Mpa to satisfy the equation σ w ≧3.38 A −¼ ; and  
       evaluating a high tensile strength structure by  
       (v) measuring a maximum inhomogeneous elemental area B of a high fatigue strength structure after said high fatigue strength structure has been  
       (a) made otherwise homogeneous by limiting the inhomogeneous elemental area, or  
       (b) minuturized by reducing a block width thereof; and  
       (vi) when said high fatigue strength has been made otherwise homogeneous by limiting the inhomogeneous elemental area, setting a distribution of a maximum-minimum range of said maximum inhomogeneous elemental area B in terms of μm within a range defined by the lines  
       (1) B ½ =0, and  
       (2) B ½ =0.9403y+4.571, wherein y is a standardizing parameter and a test standard area is 6.2×10 −9  m 2 ; and  
       (vii) when said high fatigue strength has been miniturized by reducing a block width thereof, setting a distribution of a maximum-minimum range of the block width d in terms of μm within a range defined by the lines  
       (3) d=0, and  
       (4) d=0.217y+0.701, wherein y is a standardizing parameter and a test standard area is 1.0×10 −10  m 2 .  
     
     
       22. The high fatigue strength metal according to  claim 15  wherein said metal comprises steel. 
     
     
       23. The high fatigue strength metal according to  claim 22 , wherein said flaw area A corresponds to a cross-sectional area of said inclusion.